This invention relates generally to the field of lasers, and more particularly, is directed to a semiconductor laser device having a current confinement structure and a built-in waveguide structure.
With the increased use in recent years of optical disk devices, such as digital audio discs, video discs, and "document files," and the spread of fiber optic communications, mass production of semiconductor lasers, which provide the light source for these applications, has become essential. In the manufacture of conventional semiconductor lasers, a liquid phase expitaxial process (referred to hereafter as "LPE") is used for the crystal growth on the substrate. The LPE process, however, is not suitable for mass production of semiconductor lasers for the following reasons. Firstly, the LPE process may not be used with a large size semiconductor substrate. Second, the process is not suitable for sufficiently controling crystal growth on the substrate. Finally, the process requires a substantial amount of time to complete. For these reasons, other processes for crystal growth, such as molecular beam expitaxy (referred to hereafter as "MBE") and metalorganic chemical vapor deposition (referred to hereafter as "MOCVD"), have been used for mass production of semiconductor lasers.
A semiconductor laser having a suitable construction for manufacture by the MOCVD process is disclosed in Applied Physics Letters, Vol. 37, No. 3, p. 262 and is illustrated in FIG. 1. As shown in FIG. 1, current blocking layer (5), which confines the current to the direction parallel to the junction plane and controls the transverse mode of the laser, is provided in upper clad layer (4) formed on a flat active layer (3). The active layer is formed on an n-GaAs substrate through an n-GaAlAs clad layer (2). And p-GaAlAs clad layer (4) is covered with a p-GaAs contact layer. Further, the p-GaAs contact layer and the n-GaAs substrate have a pair of metal electrodes (7), (8), respectively. This type of laser device is simple to manufacture because current confinement and transverse mode control can be realized by self alignment of the structure. This laser, however, requires a second crystal growth on the GaAlAs clad layer (4). This second crystal growth is difficult to form by the LPE process and can only be formed by the MOCVD process.
For the semiconductor laser device explained above, n-type material is used as the substrate because n-type material is better than p-type material with respect to conductivity of the current blocking layer (5). Thus, the shorter the diffusion length of the minority carriers of the current blocking layer (5), the more effective the current confinement effect is realized. It is known that for III-V type compound semiconductor material such as GaAs, the diffusion length of a hole, which is the minority carrier of the n-type semiconductor layer, is shorter than that of an electron which is the minority carrier of the p-type semiconductor layer. Therefor, the use of an n-type material for the substrate of a semiconductor laser, with the current blocking layer (5) also being n-type material, provides more effective current confinement than possible with a substrate formed from p-type material.
It has been discovered by applicants that when the above described semiconductor laser is formed so as to generate a laser beam with a lasing wave-length of 0.8 micron, the thermal characteristics of the device is poor, with a pronounced increase in lasing threshold current as the temperature rises. The maximum temperature at which the laser remains capable of oscillation is low compared with ordinary lasers which cease to oscilate at about 100.degree. C. Moreover, the working life of these semiconductor lasers is shortened under high temperature conditions.
Generally speaking, the temperature dependence I.sub.fh of the lasing threshold current I.sub.th of a semiconductor laser is expressed as follows: EQU I.sub.fh =Io exp(T/To)
In the case of an ordinary laser, the characteristic temperature To is 140 to 180 Kevin (K). The To of the laser disclosed in the Applied Physics Letters described above was found to be only 80 to 100K. This means that the temperature dependence of such a laser is considerably high. Furthermore, the current confinement effect is not sufficient. The threshold current often rises above 100 mA and the differential quantum efficiency is small, below 15%. Moreover, a so-called "kink" occurs in the current/light output characteristics when light output is on the order of 5 to 10 mw. Thus, such a laser is not suitable for high power output.